1. Field of the Invention
The present invention relates generally to a driving method for an image display medium and to an image display device, and relates more specifically to a driving method and to an image display device for an image display medium having unit cells disposed between plural substrates and at least two types of particles of different charge characteristics and color sealed inside the unit cells.
2. Description of Related Art
Twisting ball displays (dichroic twisting ball displays), electrophoretic image displays, magnetophoretic image displays, thermal rewritable image displays, and liquid crystals with memory have been proposed as display media capable of repeating image display.
Thermal rewritable display media and liquid crystals with memory stand out amongst these rewritable display media because of their excellent image memory characteristics.
Electrophoretic and magnetophoretic display media apply an electrical or magnetic field to disperse movable colored particles in a white fluid medium, and form images from the color of the colored particles and the color of the white medium. Images are formed, for example, by making the color particles adhere to the display surface in the image area to display the color of the color particles while removing the color particles from the display surface in the non-image areas to display the white of the white fluid. Furthermore, these types of displays have memory because the color particles do not move unless an electrical or magnetic field is applied.
Twisting ball displays produce an image by applying an electric field to spheric particles (balls), half of each ball being white and the other half black, to selectively reverse ball orientation. For example, the balls are driven to produce the black side to the surface of the display in the image areas, and produce the white side to the display surface in the non-image areas.
This type of display can also store an image because the balls do not change orientation unless a field is applied. This type of display medium can also be manufactured in sheets relatively easily because the inside of the display medium contains substantially solid particles, although oil is present only in the cavities around the particles.
A common problem of thermal rewritable display media and liquid crystals having memory is that a truly paper white display cannot be achieved. This means that contrast between image and non-image areas is not sufficient, and it is therefore difficult to achieve a sharp image display.
Furthermore, while the white fluid medium used in electrophoretic and magnetophoretic display media makes it possible to produce a clear white display like a paper, intrusion of the white fluid between color particles when displaying the color of the color particles causes a drop in display density. Contrast between the image and non-image areas is thus low, and it is difficult to achieve a sharp image display.
If the display medium is removed from the image display device and handled roughly like paper, there is the possibility of the white fluid sealed inside the display medium leaking.
Even when a twisting ball display is driven to produce the white spheric side of every particle to the display surface, it is not possible in principle to produce a 100% white display because ambient light rays penetrating the gaps between the balls are not reflected and are lost inside the display medium. In addition, light absorption and diffusion by the cavities mean that only a gray tinged white display can be achieved. It is also difficult to completely reverse the particles, leading to a drop in contrast and, as a result, making it difficult to produce a sharp image. Moreover, because particle size must be smaller than the pixel size, minute particles coated different colors must be manufactured in order to achieve a high resolution display. This requires sophisticated manufacturing technology.
A number of display media using toner (particles) have been proposed as a novel display medium resolving the above problems (see Japan Hardcopy L99, pp. 249-252; and Japan Hardcopy L99 fall (scheduled publication), pp. 10-13).
These display media have a transparent display substrate and a facing back substrate with a small gap therebetween, and two types of particles (toner) with different color and charge characteristics sealed in this substrate gap. When a field is applied between these substrates according to the image information, the desired color particles are made to adhere to the display substrate to form and produce an image display.
This type of particle display medium using toner also has memory because the toner does not move unless a field is applied. Such display media are also free from leakage problems because the image display medium contains nothing but solids. Furthermore, a nearly 100% reversal between white and black is in principle possible, and high contrast, sharp images can be displayed. It is also possible to display high contrast two color (such as black and white) images by using high opacity particles. It should be noted that display media using toner are referred to below as simply an image display medium.
To display multiple colors on a conventional image display medium, it is necessary, as shown in FIG. 19, to form plural unit cells inside the display medium, seal a different color of particle into each unit cell, and group a number of adjacent unit cells in order to display one color.
As shown in FIG. 19, for example, three types of unit cells, respectively containing sealed therein white and magenta particles, white and yellow particles, and white and cyan particles, are disposed in regular sequential order, and three adjacent unit cells are driven as a unit to display a particular pixel color as shown in FIG. 20.
To achieve a full color image display using this type of display medium, the white particles in each of the unit cells are collected at the display surface side to display white as shown in FIG. 20. Black is displayed by collecting the color particles in each of the unit cells to the display surface side. Magenta, yellow, and cyan are displayed by collecting the color particles to the display surface side in each unit cell containing the corresponding color particles while displaying white in the other unit cells. Red, blue, and green are displayed by driving the color particles in each unit cell so that the color particles combine appropriately as shown in FIG. 20. Note that in FIG. 20 W indicates white, M magenta, C cyan, and Y yellow.
By using plural cells to represent one pixel, this driving method leads to a drop in resolution, which is particularly apparent as a drop in text quality. It is therefore necessary to use microcells with a small surface display area per unit cell in order to maintain the resolution of display. However, it is difficult to manufacture such microcells, and when production is successful, production efficiency is poor and a rise in production cost is unavoidable.
The load on the drive circuit driving the individual cells is also high because a large number of unit cells are formed in the same display area. A high capacity drive circuit is therefore needed. An increase in drive circuit cost is therefore also unavoidable.
A further problem with a display as shown in FIG. 19 and FIG. 20 is that there is a grayish tinge to the black display, and there is a resulting drop in display quality.
It will be noted that changing the color combinations of the enclosed color particles will not change the fundamental problems of degraded text quality due to a drop in resolution, and degraded display quality due to lower contrast between black and white.
Conventional image display media are also completely reflection display media. Viewability thus drops sharply at night and in the dark, making lighting necessary. The particles used in such image display media must be highly opaque, however, and backlighting such as used with liquid crystal displays cannot be used. Such image display media must therefore use front lighting, thus limiting the expressiveness and potential application of the display medium.
The present invention was conceived with consideration for the aforementioned problems, and provides for a driving method for an image display medium and an image display device that displays multiple colors without inviting a drop in resolution, or with a suppressed drop in resolution.
This invention also provides for a driving method for an image display medium and an image display device that can achieve a high quality multiple color display.
Yet further, this invention also provides for a driving method for an image display medium and an image display device that can use backlight drive.
The present invention has been made in view of the above circumstances and provides a driving method for an image display medium, the image display medium having plural facing substrates of which at least a display-side substrate is transparent, a unit cell delimited between the substrates, and at least two types of particle groups of different color and charge characteristics sealed inside the unit cells. The driving method has steps of: producing a color display by applying a particle drive voltage pulse of a saturation voltage, which is high enough to saturate the color density of the at least one particle group of the at least two particle group types that moves to the display-side substrate as a result of a field produced by applying voltage to a unit cell; and producing a transparent display by applying a particle drive voltage pulse of an absolute voltage greater than the saturation voltage so that the two particle group types inside the unit cells agglomerate and make the unit cell transparent.
The present invention further provides an image display device having: plural facing substrates of which at least a display-side substrate is transparent; a support member disposed between each of the plural substrates and delimiting a unit cell; particle groups of at least two types sealed in the unit cell, having different color and charge characteristics, and moving in mutually opposite directions between the substrates in response to an applied field; a pair of electrodes disposed between plural facing substrates to form a field in the unit cell; and a display control unit for controlling so as to display the color of at least one of the particle groups by applying a particle drive voltage pulse of a saturation voltage, which is high enough so that the color of the at least one particle group of the at least two particle groups that moves to the display-side substrate due to the applied field reaches a saturation density, and controlling so as to produce a transparent display by applying a particle drive voltage pulse of an absolute voltage greater than the saturation voltage so that the two particle group types inside the unit cells agglomerate and make the unit cell transparent.
While conducting repeated particle drive tests in an image display device that displays color at a transparent display substrate by moving one of two different types of particles sealed inside a unit cell to a transparent display-side substrate, the inventors discovered the following. That is, if the absolute value of the particle drive voltage is greater than the particle drive voltage at which color saturation is achieved (referred to below as the saturation voltage), the sealed particles agglomerate. The surface area occupied at the display surface by these particle clumps is extremely small. As a result the unit cells are apparently transparent, and the back side of the unit cells can be seen from the display surface.
Furthermore, the inventors also discovered the following through repeated experiments with agglomeration of the sealed particles. That is, if the frequency of the particle drive voltage pulse applied for agglomeration is higher than the frequency of the particle drive voltage pulse applied when displaying color using the color of the particle groups, the time required for the particle groups to agglomerate and the unit cell to become transparent is shortened.
Furthermore, if the frequency of the particle drive voltage pulse applied to dissociate the agglomerated particle groups and make the transparent unit cells no longer transparent is higher than the frequency of the particle drive voltage pulse applied to achieve a color display, the time required for the particle groups to dissociate and redisperse in the unit cell is shortened.
As a result of these findings, the present invention applies a particle drive voltage pulse at a repeat frequency higher than the repeat frequency of the particle drive voltage pulse at the saturation voltage to produce a transparent display. This makes it possible to change quickly from a color display to a transparent display.
Furthermore, to end the transparent display, the present invention applies a particle drive voltage pulse of a voltage lower than the voltage causing particle groups to agglomerate at a frequency higher than the repeat frequency of the particle drive voltage pulse at the saturation voltage. This makes it possible to change quickly from a transparent display to a color display.
Yet further, because the inside of a unit cell can be seen from the display side when the unit cell is made transparent, the present invention can be configured to display three colors using the particle colors and a back substrate color by making the back substrates a different color than the colors of the two particle types.
Furthermore, if the image display device has a laminated configuration with three or more substrates, and plural stacked unit cells disposed between substrates are driven as one display cell, a multiple color display can be produced using the substrate color and the colors of the particles sealed inside the plural unit cells.
Further alternatively, an image display device according to this invention has a light emitting unit for emitting light toward the display-side substrate. In this case all substrates are transparent and the light emitting unit is disposed behind the back substrate. Because light passes from unit cells set to a transparent state, a clear image display can be achieved even in dark surroundings, and a high contrast image display can be achieved.